Very long optical fiber transmission paths, such as those employed in undersea or trans-continental terrestrial lightwave transmission systems that employ optical amplifier repeaters, are subject to decreased performance due to a host of impairments that increase as a function of the length of the optical fiber comprising the transmission path. Typically, in such long optical transmission systems, these impairments vary with time and cause a random fluctuation in the signal-to-noise ratio ("SNR") of the transmission path. This random fluctuation contributes to a phenomenon known as signal fading. Signal fading can result in an increased bit error rate ("BER") for digital signals transmitted via the optical fiber path. When the SNR of a digital signal within such a transmission system becomes unacceptably small (resulting in an undesirably high BER), a signal fade is said to have occurred. Experimental evidence has shown that signal fading and SNR fluctuations are caused by several polarization dependent effects induced by the optical fiber itself and/or other optical components (e.g., repeaters, amplifiers, etc.) along the transmission path. In general, these effects result from the anisotropic transmission characteristics of the optical components and fiber within a transmission system. For example, anisotropic loss or gain as a function of signal polarization.
Doped fiber amplifiers (such as the erbium-doped fiber amplifiers typically employed as repeaters within long-haul optical fiber transmission systems) can exhibit anisotropic gain as signal polarization is varied. This is a newly identified mechanism of transmission anisotropy called polarization dependent gain ("PDG"). PDG has two components; one is associated with the signal passing through the doped fiber amplifier, and the other with the optical pump signal used to excite the doped fiber amplifier. To distinguish between these two, the latter is designated PDG.sub.PUMP. PDG.sub.PUMP is the active component counterpart to polarization dependent loss ("PDL"), a well-known phenomenon that causes anisotropic losses within passive components. PDG.sub.PUMP arises from the random orientation of dopant ions within the glass matrix of the fiber amplifier, and the selective excitation of these ions by a linearly polarized pump laser. The gain of the doped fiber amplifier induced by this pumping is larger for signals having a linear state of polarization ("SOP") parallel to the SOP of a linearly polarized pumping signal. Ideally, all PDG.sub.PUMP should be removed from paths within optical transmission systems to achieve optimum performance. Unfortunately, standard techniques used to remove or reduce PDL in passive system components prove ineffective in eliminating PDG.sub.PUMP.